Optical information detection apparatus

ABSTRACT

An optical information detecting apparatus includes a first optical system focusing an optical beam on a recording surface of a recording medium and a second optical system directing a reflection optical beam produced as a result of reflection of the optical beam by a recording surface of the recording medium to a photodetection unit, wherein the second optical system including a beam dividing element disposed so as to intercept the reflection optical beam and divide the reflection beam into a plurality of optical beam elements traveling generally parallel with each other in the reflection optical beam, such that the plurality of optical beam elements reach the photodetection unit along respective optical paths.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to reading of informationfrom an optical recording medium and more particularly to a compact andhigh-density optical information detection apparatus capable ofreproducing information from a high-density optical recording medium inwhich information is recorded on both a land and a groove that define atrack. More specifically, the present invention relates to an opticalinformation detection apparatus in which cross-talk between theinformation read out from the land and read out from the groove isminimized and wherein the resolution at the time of detection of therecorded information is improved.

[0003] Optical disks are used extensively as the recording medium ofvarious information including audio and visual data. In relation to theart of high-density rewritable recording of information, intensiveefforts are being made particularly with regard to the development ofrewritable optical disks such as a magneto-optical disk or a phasetransition disk.

[0004] In order to increase the recording density of such optical disks,it is desired to decrease the wavelength of the optical beam used forinformation detection or to increase the numerical aperture of theobjective lens such that the beam spot of the optical beam on therecording medium is reduced.

[0005] Further, there is a proposal to use an MSR (magneticsuper-resolution) technology. It should be noted that the MSR technologyattempts to increase the recording density of a magneto-opticalrecording medium while using the optical beam of the same spot size, bysuppressing the cross-talk between the tracks or between the recordingmarks aligned in the tangential direction of the track as much aspossible. However, the MSR technology still includes various problemsrelated to resolution which appear conspicuously when the track pitch isreduced, such as the decrease of tracking performance or the increase ofthe crosstalk. In the case of a rewritable optical disk such as amagneto-optical disk, the cross-erasing of information becomes also aserious problem.

[0006] Meanwhile, there is a proposal of so-called land-groove recordingtechnology that increases the effective track recording density twice ascompared with the conventional land recording technology or grooverecording technology. In the conventional land recording technology orgroove recording technology, the information is recorded only on theland or on the groove that defines a track, while the information isrecorded both on the land and the groove in the land-groove recordingtechnology.

[0007] In the land-groove recording technology, in which lands andgrooves are separated three-dimensionally, the problem of cross-erasingof information is effectively suppressed as a result of the spatialseparation of the lands and the grooves. Thus, the land-groove recordingtechnology is thought an effective approach to increase the recordingdensity of optical disks including rewritable optical disks. In order toreduce this promising technology into practice, however, it is necessaryto device a method of suppressing the cross-talk further.

[0008] 2. Description of the Prior Art

[0009] Conventionally, there is a proposal to reduce the cross-talk asdescribed in the Japanese patent application 9-16134, wherein this priorapplication achieves the desired suppressing of the cross-talk betweenthe lands and the grooves by applying a phase compensation to theoptical signals produced by the lands and produced by grooves of theoptical recording medium independently. When the desired increase of theline recording density is to be achieved according to this priorapplication while using the same spot size for the optical beam, on theother hand, there is a need of a further process for compensating forthe decrease of the reproduced signal output. It should be noted thatsuch a decrease of the reproduced signal output is caused by theinterference of the recording marks aligned on a track.

[0010] With regard to the improvement of resolution of the reproducedsignal output for the recording marks aligned on a track, there is aproposal of optical super-resolution by Milster, T. D., et al., JapaneseJ. Appl. Phys. vol.32, 1993, pp.5397-5401, in which a shading band isprovided in the optical path which is used for detecting the informationfrom an optical disk. Thereby, the shading band functions as an opticalequalizer.

[0011] Further, in view of the recent trend of technology that targetsan integrated optical head carrying a hologram, it is desired that thehigh-density recording method is compatible with the construction ofsuch integrated optical heads.

[0012] Furthermore, there is a proposal of optical information detectionmethod as disclosed in the Japanese Laid-Open Patent Publication9-128825, in which simultaneous detection of different information isachieved by dividing a reflected optical beam into several optical beamsby using one or more optical beam splitters. It should be noted that theprocess of this prior art achieves the optical beam splitting withrespect to the entirety of the optical beam, by disposing the opticalbeam splitter so as to intercept the entire optical beam that isreflected by the optical recording medium and traveling toward anoptical detection system.

[0013] With regard to the process of the Japanese patent application9-16134 noted before, it is confirmed that the MSR process is aneffective approach for suppressing the cross-talk between the tracks andthe interference between the recording marks aligned on a track. On theother hand, the process of the foregoing prior application has adrawback in that it requires at least two magneto-optical layers on themagneto-optical recording medium and that a high optical power has to beused for the optical beam used for reading information. Further, thereis an additional drawback in that an exact control the of the opticalbeam power is necessary such that the optical beam power falls within anarrow tolerance range.

[0014] In addition to the foregoing, the process of the Japanese patentapplication 9-16134 has a drawback in that, while the problem of thecross-talk between the tracks may be successfully reduced, thereproduced optical beam tends to have an ecliptic polarization state dueto the admixing of polarization components having a mutual phase offsetcorresponding to twice the depth of the groove, into the reflectedoptical beam. It should be noted that such an admixing of thepolarization component occurs as a result of the reflection of theoptical beam at the land and the groove adjacent thereto. When thisoccurs, the output of the reproduced signal is deteriorated inevitably.In order to avoid this problem, it is necessary to provide anappropriate optical phase compensation process.

[0015] It is possible to achieve the desired increase of the trackrecording density and the linear recording density without using the MSRtechnology, by combining the optical super-resolution of the Milster etal., op. cit., which uses a shading band, with the optical phasecompensation process applied separately to the optical beams reflectedfrom the lands and reflected from the grooves. However, such a processrequires a construction in which the optical shading band and theoptical phase compensation device are provided for each of the opticalbeams reflected by the lands and the grooves, and the construction ofthe optical system becomes inevitably bulky and complex.

[0016] In the process of Japanese Laid-Open Patent Publication 9-128825,which divides the reflected optical beam into a plurality of opticalbeam elements, on the other hand, there has been a problem in that it isdifficult to construct the optical information detection apparatus tohave a compact size, due to the fact that the beam splitting is appliedto the entirety of the reflected optical beam at several locations ofthe optical path of the reflected optical beam and that it is necessaryto provide a detection optical system to each of the optical beam thusdivided.

SUMMARY OF THE INVENTION

[0017] Accordingly, it is a general object of the present invention toprovide a novel and useful optical information detection apparatuswherein the foregoing problems are eliminated.

[0018] Another and more specific object of the present invention is toprovide a compact and efficient optical information detection apparatusthat is capable of detecting various different information recorded onan optical recording medium.

[0019] Another object of the present invention is to provide an opticalinformation detection apparatus, comprising:

[0020] a turn-table adapted for holding an optical disk thereonrotatably, said optical disk including a land and an adjacent grooveformed on a surface thereof, both of said land and groove carryingrespective information;

[0021] a motor connected to said turn-table so as to rotate saidturn-table;

[0022] an optical source emitting an optical beam;

[0023] a first optical system directing said optical beam from saidoptical source to said optical disk held in a state that said opticaldisk is held on said turn-table;

[0024] a second optical system collecting and guiding a reflectionoptical beam produced by a reflection of said optical beam at saidsurface of said optical disk in a state that said optical disk is heldon said turn-table;

[0025] a beam dividing element dividing said reflection optical beaminto a plurality of optical beam elements each corresponding to a partof said reflection optical beam and traveling side by side in saidreflection optical beam; and

[0026] a plurality of photodetection devices respectively detecting saidplurality of optical beam elements.

[0027] According to the present invention, the reflection optical beamis divided into a plurality of optical beam elements each correspondingto a part of the reflection optical beam and traveling generally side byside as forming the reflection optical beam, wherein the plurality ofoptical beam elements carry respective, specific information such astracking error information, focusing error information, the recordedinformation recorded on the groove, and the recorded informationrecorded on the land. Thereby, it is possible to extract various opticalinformation from the reflection optical beam by a simple constructionand the size of the optical information detection apparatus can bereduced successfully.

[0028] Other objects and further features of the present invention willbecome apparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a diagram showing the construction of an opticalinformation detection apparatus according to an embodiment of thepresent invention;

[0030]FIG. 2 is a diagram showing the construction of an opticalrecording medium used in the optical information detection apparatus ofFIG. 1;

[0031]FIG. 3 is another diagram showing the construction of the opticalrecording medium of FIG. 2 in detail;

[0032]FIGS. 4A and 4B are diagrams showing a composite optical elementused in the optical information detection apparatus of FIG. 1;

[0033]FIG. 5 is a diagram showing the composite optical element of FIGS.4A and 4B in an exploded view;

[0034]FIG. 6 is a diagram showing the construction of a photodetectorarray used in the optical information detection apparatus of FIG. 1;

[0035]FIG. 7 is a circuit diagram showing a detection circuit used inthe optical information detection apparatus of FIG. 1 for reading theinformation from the optical recording medium;

[0036]FIG. 8 is another circuit diagram showing another detectioncircuit used in the optical information detection apparatus of FIG. 1together with the circuit of FIG. 7;

[0037]FIG. 9 is a diagram showing a diffraction pattern of a reflectionoptical beam obtained by the composite optical element of FIGS. 4A and4B; and

[0038]FIG. 10 is a diagram showing a jitter of a recording mark formedon the optical recording medium used in the optical informationdetection apparatus of FIG. 1 for various recording optical power.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039]FIG. 1 shows the construction of an optical information recordingand reproducing apparatus in which an optical information detectionapparatus of the present invention is used.

[0040] Referring to FIG. 1, there is provided a laser diode 1 thatproduces an optical beam, wherein the optical beam is converted, afterpassing through a collimator lens 2 and a beam shaping prism 3, to aparallel optical beam L₁ having a circular beam cross-section. Theoptical beam L₁ is then directed to a polarization beam splitter 4 thatdivides the optical beam L₁ to form an optical beam L₂ and an opticalbeam L₃, wherein the optical beam L₂ is directed to a photodetector 5for automatic optical power control.

[0041] On the other hand, the optical beam L₃ is directed to amagneto-optical disk 7 and is focused on a surface thereof by anobjective lens 6. It should be noted that the magneto-optical disk 7 isheld on a turn-table by a chuck mechanism CH and is rotated by a spindlemotor SP at a high speed. Further, it should be noted that the objectivelens 6 is held movably on a biaxial actuator not illustrated such thatthe objective lens is movable in a radial direction of the disk 7 andfurther in a direction to and from the disk 7. As noted already, theobjective lens focuses the optical beam L₃ to a desired point on arecording surface of the magneto-optical recording disk 7 on whichconcentric or spiral-shaped guide tracks are formed.

[0042]FIG. 2 shows the construction of the magneto-optical recordingdisk 7.

[0043] Referring to FIG. 2, the magneto-optical recording disk 7 has acentral hub 7 b and is accommodated in a case 7 a, wherein a spiraltrack TR is formed on a recording surface of the disk 7 that faces theobjective lens 3 for a tracking servo control of the objective lens 6.The spiral track TR is defined by a spiral-shaped or concentric-shapedgroove and an adjacent, spiral-shaped or concentric-shaped land.

[0044]FIG. 3 shows the recording surface of the magneto-opticalrecording disk 7 in an enlarged scale.

[0045] Referring to FIG. 3, each of the tracks TR is defined by a groove7 e and a land 7 e, and the surface of the disk 7 is covered by amagnetic recording film 7 f. Further, a floating magnetic head 8 isdisposed at the opposite side of the recording surface of themagneto-optical disk 7 as indicated in FIG. 1.

[0046] In a write mode operation of the optical recording andreproducing apparatus of FIG. 1, the magnetic recording film 7 f isheated locally by the optical beam L₃ that is focused on the recordingsurface of the disk 7 with a large optical power. As a result of such alocalized heating, the direction of magnetization of the magnetic film 7f is rotated according to the magnetic field of the magnetic head 8 andthe writing of the information is achieved thereby.

[0047] In a read mode operation, on the other hand, the optical beam L₃is focused on the recording surface of the magneto-optical disk 7 with areduced optical power, and the plane of polarization of the opticalbeam. L₃ is rotated according to the direction of magnetization of themagnetic film 7 f as the optical beam L₃ is reflected by the magneticfilm 7 f. The optical information recording and reproducing apparatus ofFIG. 1 thereby detects the content of the recorded information bydetecting the polarization state of the reflected optical beam.

[0048] More specifically, a reflected optical beam L₄ thus produced as aresult of the reflection of the optical beam L₃ at the magneto-opticaldisk 7 returns to the polarization beam splitter 4 after passing throughthe objective lens 6 in a reverse direction, wherein the polarizationbeam splitter 4 reflects the reflection optical beam L₄ thus returnedthereto to a composite optical element 9 as a reflection optical beamL₅. See FIG. 1.

[0049] As will be described below, the composite optical element 9decomposes the reflection optical beam L₅ into respective optical beamelements representing the recorded information, focusing errorinformation and tracking error information, wherein the optical beamelements thus produced are focused by a lens 10 on a photodetector array11.

[0050]FIGS. 4A and 4B show the construction of the composite opticalelement 9 respectively in a front view and a side view. The samecomposite optical element 9 is shown also in FIG. 5 in an exploded view.

[0051] Referring to FIGS. 4A and 4B, the composite optical element 9includes retardation plates 12 and 13 disposed adjacent with each otherside by side in the A₁-A₂-direction, wherein the retardation plates 12and 13 carry thereon a first Wollaston prism 14 a and a second Wollastonprism 14 b respectively. As indicated in FIG. 4A, the retardation plates12 and 13 have an overall size corresponding to a beam size do of anincident optical beam 18 which corresponds to the reflection opticalbeam L₅, and the Wollaston prisms 14 a and 14 b are separated from eachother in a lateral direction of the optical element 9 with a distanced₁. The Wollaston prism 14 a carries a wedge prism 15 a thereon and theWollaston prism 14 b carries a wedge prism 15 b.

[0052] Further, there is disposed a double-wedge prism 17 formed ofwedge prisms 17 a and 17 b, wherein the wedge prisms 17 a and 17 b haverespective prism surfaces inclined in mutually opposite directions on asubstrate formed by the foregoing retardation plates 12 and 13 and aredisposed at a central part defined by the Wollaston prisms 14 a and 14 bas indicated in FIG. 4B or FIG. 5. Further, the double-wedge prism 17 islaterally sandwiched by wedge prisms 16 a and 16 b having mutuallyinclined wedge surfaces in the B₁-B₂-direction.

[0053] As indicated in FIGS. 4A and 4B, the incident optical beam 18corresponding to the reflection optical beam L₅ is directed to thebottom of the substrate formed of the retardation plates 12 and 13,wherein the retardation plate 12 applies an optical phase compensationto an optical beam element 18A reflected by the land 7 d of the opticaldisk 7 of FIG. 1 and forming a part of the optical beam 18 such That thepertinent optical beam element 18A has a predetermined optical phase. Onthe other hand, the retardation plate 13 applies an optical phasecompensation to an optical beam element 18B reflected by the groove 7 eof the optical disk 7 of FIG. 1 and forming a part of the optical beam18, wherein it should be noted that the optical phase compensationachieved by the retardation plate 13 has a magnitude substantiallyidentical to the optical phase compensation achieved by the retardationplate 12 but the direction of the optical phase compensation of theretardation plate 13 is set opposite to the direction of the opticalphase compensation achieved by the retardation plate 12.

[0054] The optical beam element 18A thus passed through the retardationplate 12 is then caused to enter the Wollaston prism 14 a, wherein theoptical beam element 18A, the optical phase of which is compensated bythe retardation plate 12, experiences a deflection, inside the Wollastonprism 14 a, in one of the B₁- and B₂-directions depending on thepolarization state of the optical beam element. Similarly, the opticalbeam element 18B passed through the retardation plate 13 enters theWollaston prism 14 b and experiences a deflection inside the Wollastonprism 14 b in one of the B₁- and B₂-directions depending on thepolarization state of the optical beam element. See FIG. 4B.

[0055] The optical beam element 18A thus deflected by the Wollastonprism 14 a is then caused to pass through the wedge prism 15 a, whereinthe wedge prism 15 a, having a prism surface inclined in theA₁-direction, refracts the optical beam element 18A in the A₁-direction.Similarly, the optical beam element 18B thus deflected by the Wollastonprism 14 b is caused to pass through the wedge prism 15 b, and the wedgeprism 15 b, having a prism surface inclined in the A₂-direction,refracts the optical beam element 18B in the A₂-direction.

[0056] In addition, the wedge prism 16 a deflects a marginal ray 18 aincluded in the reflection optical beam 18 in the B₁-direction and anopposite marginal ray 18 b also included in the reflection optical beam18 in the B₂-direction. The marginal rays 18 a and 18 b thus deflectedby the wedge prisms 16 a and 16 b are collected by the lens 10 andfocused on the photodetector array 11 for the detection of a push-pulltracking error signal. Further, the optical beam elements 18A and 18Bare collected by the lens 10 and are focused on the photodetector array11 for differential detection of the recorded magneto-opticalinformation signal.

[0057] The double-wedge prism 17 is disposed so as to intercept the corepart of the reflection optical beam 18. As noted already and asindicated in FIG. 5, the double-wedge prism 17 is formed of two wedgeprisms 17 a and 17 b disposed side by side, wherein the wedge prisms 17a and 17 b have respective, mutually oppositely inclined prism surfaces.

[0058] More specifically, the first wedge prism 17 a is disposed on theretardation plate 12 in alignment with the edge thereof at the side ofthe A₂-direction, while the second wedge prism 17 b is disposed on theretardation plate 13 in alignment with the edge thereof at the side ofthe A₁-direction. Thereby, the wedge prism 17 a causes a deflection ofan incident optical beam in the B₁-direction while the wedge prism 17 bcauses a deflection of an incident optical beam in the B₂-direction.

[0059] Thus, the double wedge prism 17 decomposes a core part 19 of thereflection optical beam 18 into two optical beam elements 19 a and 19 b,wherein the optical beam elements 19 a and 19 b are forwarded to thephotodetection array 11 via the lens 10 for extracting a focusing errorsignal by a double Foucault process.

[0060] As noted already, the optical beam element forming the reflectionoptical beam 18 at the side of the core part 19 is subjected to anoptical phase compensation process achieved independently by theretardation plate 12 and the retardation plate 13, and each of theoptical beam elements is deflected in one of the B₁-B₂-directions by theWollaston prism 14 a or 14 b according to the polarization statethereof. The optical beam elements thus separated are directed to thephotodetector array 11 at the wedge prism 15 a or 15 b for detection ofthe recorded signal.

[0061] Hereinafter, the reflection occurring at the magneto-opticalrecording medium 7 will be explained.

[0062] As described previously, the optical beam L₃ illuminates both theland 7 d and the groove 7 e adjacent to the land 7 e, and thus, theoptical beam 18 corresponding to the reflection optical beam H₄inevitably includes a cross-talk component.

[0063] In the case of the magneto-optical disk 7, the land 7 d and thegroove 7 e are formed with a step height corresponding to one-eighth thewavelength of the optical beam 18, such that there appears a phaseoffset corresponding to one-quarter the wavelength between the opticalbeam element 18A reflected by the land 7 d and the optical beam element18B reflected by the groove 7 e. Because of the superposition of theoptical beam components 18A and 18B thus shifted in phase in the opticalbeam 18, there inevitably appears a cross-talk between the optical beamcomponents 18A and 18B, and each of the optical beams 18A and 18Bbecomes an elliptically polarized light. Thereby, the direction ofrotation of the polarization ellipse for the optical beam component 18Abecomes opposite to that of the optical beam component 18B. This alsomeans that it is possible to suppress the detection of unwanted opticalbeam component by setting the retardation plates 12 and 13 such that anoptimum optical phase compensation is achieved separately for each ofthe land 7 d and the groove 7 e.

[0064] Thus, in the present embodiment, the retardation plate 12 is setsuch that the cross-talk of the optical beam component 18B to theoptical beam component 18A becomes minimum and the retardation plate 13is set such that the cross-talk of the optical beam component 18A to theoptical beam component 18B becomes minimum.

[0065] It should be noted that the composite optical element 9 of FIGS.4A and 4B merely represents an example, and the composite opticalelement 9 may be formed as an integral unitary body.

[0066] Hereinafter, the construction of the photodetector array 11 willbe described with reference to FIG. 6.

[0067] Referring to FIG. 6, the photodetector array 11 is formed on acommon substrate 11 a that carries thereon first through seventh opticaldetectors 20-26, wherein the optical detectors 20 and 21 are disposed inalignment in the B₁-B₂-directions at a side of the substrate 11 a in theA₂-direction. Thereby, it should be noted that the optical detector 20is disposed so as to receive the optical beam component 18B deflected bythe Wollaston prism 14 b in the B₁-direction, while the optical detector21 is disposed so as to receive the optical beam component 18B deflectedby the Wollaston prism 14 b in the B₂-direction.

[0068] Further, it can be seen that the substrate 11 a of thephotodetector array 11 carries thereon the optical detectors 22 and 23in alignment in the B₁-B₂-direction at a size of the substrate 11 a inthe A₁-direction. Thereby, it should be noted that the optical detector22 is disposed so as to receive the optical beam component 18A deflectedby the Wollaston prism 14 a in the B₁-direction, while the opticaldetector 23 is disposed so as to receive the optical beam component 18Adeflected by the Wollaston prism 14 a in the B₂-direction.

[0069] Further, the substrate 11 a of the photodetector array 11 carriesthereon a central photodetection part 24 at the central part of thesubstrate 11 a, wherein the photodetection part 24 includes fourphotodetection regions 24 a-24 d in correspondence to four quadrants.

[0070] Furthermore, the substrate 11 a of the photodetector array 11carries thereon photodetectors 25 and 26 at both sides of the centralphotodetection part 24 in the B₁-B₂-directions, wherein thephotodetector 25, located at the side of the B₁-direction of the centralphotodetection part 24, detects the optical beam deflected by the wedgeprism 16 a in the B₁-direction. Further, the photodetector 26 at theB₂-side of the photodetection part 24 detects the optical beam deflectedby the wedge prism 16 b in the B₂-direction.

[0071] In the photodetector array 11 of FIG. 6, it should be noted thatthe information recorded on the land 7 d of the optical disk 7 isreproduced by obtaining a difference between a detection signal detectedby the photodetector 20 and a detection signal detected by thephotodetector 21. On the other hand, the information recorded on thegroove 7 e of the optical disk is reproduced by obtaining a differencebetween a detection signal detected by the photodetector 22 and adetection signal the photodetector 23.

[0072] On the other hand, a focusing error signal is obtained byapplying a predetermined operation to be described below to thedetection signals obtained by the photodetectors 24 a-24 d forming thecentral photodetection part. Further, a tracking error signal isobtained by obtaining a difference between the detection signal detectedby the photodetector 25 and the detection signal detected by thephotodetector 26.

[0073] Hereinafter, a more detailed description will be made on theconstruction for reproducing the recorded information from the land 7 dand from the groove 7 e of the optical disk 7 as well as theconstruction for extracting the tracking error signal and the focusingerror signal.

[0074]FIG. 7 shows the construction for reproducing the recordedinformation from the land 7 d and the groove 7 e as well as theconstruction for obtaining the tracking error signal.

[0075] Referring to FIG. 7, there is provided a first differentialamplifier Amp1 such that a non-inverting input terminal thereof isconnected to the first photodetector 20 and an inverting input terminalthereof connected to the second photodetector 21. Thereby, thedifferential amplifier Amp1 produces an information signal correspondingto the information recorded on the land 7 d of the optical disk 7 as thedifference between the output of the photodetector 20 and thephotodetector 21. It should be noted that the photodetectors 20 and 21receive the optical beam element 18A of which optical phase iscompensated by the retardation plate 13 such that the cross-talk fromthe groove 7 e is minimized.

[0076] Further, there is provided a second differential amplifier Amp2such that a non-inverting input terminal thereof is connected to thephotodetector 22 and an inverting input terminal thereof connected tothe photodetector 23. Thereby, the differential amplifier Amp2 producesan information signal corresponding to the information recorded on thegroove 7 e of the optical disk 7 as the difference between the output ofthe photodetector 22 and the photodetector 23. It should be noted thatthe photodetectors 22 and 23 receive the optical beam element 18B ofwhich optical phase is compensated by the retardation plate 12 such thatthe cross-talk from the land 7 d is minimized.

[0077]FIG. 7 further shows another differential amplifier Amp3 having aninverting input terminal connected to the photodetector 25 and anon-inverting input terminal connected to the photodetector 26, whereinthe differential amplifier Amp3 produces the tracking error signal as adifference between the output of the photodetector 26 and the output ofthe photodetector 25. It should be noted that the photodetectors 25 and26 receive the reflected optical beams from the land 7 d and the groove7 e, wherein the photodetectors 25 and 26 receive the same amount ofoptical radiation when the center of the optical beam H₃ used forreading the information is located exactly on the boundary of the land 7d and the groove 7 e. In such a case of ideal tracking, thephotodetectors 25 and 26 produce the same output signal and the outputof the differential amplifier Amp 3 becomes zero.

[0078] When there is a deviation in the tracking, on the other hand,there appears a difference in the optical beam intensity between theoptical beam received by the photodetector 25 and the optical beamreceived by the photodetector 26. For example, the intensity of theoptical beam received by the photodetector 26 may decrease when theintensity of the optical beam received by the photodetector 25 isincreased, or vice versa. Thus, when the output of the photodetector 25is increased, the output of the photodetector 26 is decreased and thedifferential amplifier Amp3 produces a negative output. In the oppositecase, the differential amplifier Amp3 produces a positive output. Thus,the differential amplifier Amp3 produces an output signal indicative ofthe tracking state as the tracking error signal.

[0079]FIG. 8 shows a construction used for detecting a focusing errorsignal.

[0080] Referring to FIG. 8, there is provided a summation amplifier Amp4connected across the photodetector 24 a and the photodetector 24 caligned in a diagonal direction in the central photodetection part 24,and another summation amplifier Amp 5 is connected across thephotodetector 24 b and the photodetector 24 d aligned also in anotherdiagonal direction of the central photodetection part 24. Wherein thesummation amplifiers Amp4 and Amp5 produce an output indicative of asummation of the input signals supplied thereto. Further, there isprovided a differential amplifier Amp6 having a non-inverting inputterminal to which the output of the summation amplifier Amp4 is suppliedand an inverting input terminal to which the output of the output of thesummation amplifier Amp5 is supplied, wherein the summation amplifiersAmp4 and Amp5 carry out, together with the differential amplifier Amp6,a focusing detection according to a double Foucault process.

[0081] More in detail, the double Foucault process utilizes the natureof the reflection optical beam that the reflection optical beam has acircular beam shape when the optical beam is focused properly on thereflection surface. In such a properly focused state, therefore, thephotodetectors 24 a-24 d produce a generally identical output and thedifferential amplifier Amp6 produces a zero output.

[0082] When the focusing state of the optical beam is offset from theproperly focused state, on the other hand, there is a tendency that thephotodetectors 24 a and 24 c receive an increased optical radiation andthe photodetectors 24 b and 24 d receive a decreased optical radiation,or vice versa. In such a state therefore, the summation amplifier Amp4produces a larger output and the summation amplifier Amp5 produces asmaller output, or vice versa, and the output of the differentialamplifier Amp6 is no longer zero. In fact, the differential amplifierAmp6 produces a positive output when the output of the summationamplifier Amp4 is increased and the output of the summation amplifierAmp5 is decreased or a negative output when the output of the summationamplifier Amp5 is increased and the output of the summation amplifierAmp4 is decreased. Thus, the focusing error signal is obtained as anoutput of the differential amplifier Amp6.

[0083]FIG. 9 shows a diffraction pattern obtained in the reflectionoptical beam L₄ reflected by the optical disk 7. Because of the presenceof the land and grooves repeated with a periodical pitch, it should benoted that the optical beam L₃ focused on the recording surface of theoptical disk 7 experiences a diffraction and the reflection optical beamL₄ produced as a reflection of the optical beam L₃ shows the diffractionpattern as indicated in FIG. 9.

[0084] Referring to FIG. 9, the diffraction pattern includes aband-shaped zeroth-order diffraction beam 27 having a width 30 and twofirst-order diffraction beams 28 and 29 at both sides of thezeroth-order beam 27, wherein the zeroth-order beam 27 does not carryinformation of the recording mark in the optical recording andreproducing apparatus of FIG. 1 in which the recording mark has a lengthsmaller than the size of the beam spot. Thus, the resolution of theoptical information detection in the optical recording and reproducingapparatus of FIG. 1 can be improved by cutting off the zeroth-orderdiffraction beam 27.

[0085] In the present invention, rather than providing a shading bandcontrary to the teaching of Milster et al., op cit., the Wollastonprisms 14 a and 14 b are separated from each other with the separationd₁ set coincident with the zeroth-order diffraction beam 27, and thefirst-order diffraction beams 28 and 29 are processed by the Wollastonprisms 14 a and 14 b and directed to the photodetectors 20 and 21 or tothe photodetectors 22 and 23 for information detection. Further, thezeroth-order beam 27 is used also effectively for the tracking controland the focusing control explained before, by providing the wedge prisms16 a, 16 b and 17 a, 17 b in correspondence to the optical path of thezero-th order beam 27.

[0086] In the construction of the magneto-optical recording andreproducing apparatus of FIG. 1, it should be noted that a laser diodeproducing an output optical beam with a wavelength of 650 nm is used forthe laser diode 1, and a lens having a numerical aperture of 0.6 is usedfor the objective lens 6. Further, the direction of polarization of thereflection optical beam H₄ reflected by the optical disk 7 is setgenerally parallel to the elongating direction of the land 7 d and hencethe groove 7 e. In the case the direction of polarization is setperpendicular to the elongating direction of the land 7 d and the groove7 e, a 1/T wavelength plate may be used for rotating the polarizationplane by 90° in the optical detection system.

[0087] In the magneto-optical disk 7, a glass disc having a thickness of0.6 mm may be used for the substrate 7 c, and the land 7 d and thegroove 7 e may be formed with a tack pitch of 1.2 μm (effective trackpitch of 0.6 μm) by using a photo-polymer forming process. On thesubstrate 7 c, the recording film 7 f is formed as a four-layer stackingstructure including a dielectric layer, a magneto-optical recordinglayer, another dielectric layer and a metal reflection layer. Therecording film 7 f may be formed by a sputtering process and is coveredby a protective film of a UV-cure resin with a thickness of severalmicrons. As described previously, the groove 7 e is formed to have adepth corresponding to one-eighth (⅛) the wavelength of the laser beamused for reading information.

[0088] It should be noted that the material of the substrate 7 c is byno means limited to a glass disc noted above but an injection moldedplastic disc of polycarbonate, and the like, may also be used as long asthe plastic disc has a small warp and little birefringence.

[0089] In the present embodiment, the recording film 7 f may include anamorphous alloy film of TbFeCo as the magneto-optical recording layer.When the TbFeCo alloy is used for the magneto-optical recording layer,the recording film 7 f provides a Kerr rotation angle of 0.9°, a Kerrellipticity of 0° and a reflectance of 18%, including the contributionfrom the four-layer structure. Further, a multilayer film for MSR beused for the magneto-optical film 7 f.

[0090] The writing of information onto the magneto-optical disk 7 may beachieved by using a floating magnetic head that creates a modulationmagnetic field in combination with a pulse-assisted magnetic modulationprocess in which a laser pulse is applied in synchronization with thewriting of information. While it is possible to carry out the recordingof information by a magnetic field modulation process that uses a DClaser beam or by an optical modulation process, the use of thepulse-assisted magnetic modulation process is preferred in view ofimproved quality of reproduced signal output.

[0091] In FIG. 4A, it should be noted that the width d₁ of the compositeoptical element 9 is optimized in the state that the retardation plates12 and 13 are not provided, by measuring the signal strength whilecutting the reflection beam by a knife edge. In the illustrated example,the width d₁ is set to 1.5 mm for the beam diameter d₀ of 6 mm.

[0092] Further, it should be noted that the optimum value of theretardation of the retardation plates 12 and 13 is obtained by readingout the information signal from the land 7 d and the groove 7 e whilechanging the retardation variously by using a Babinet-Soleilcompensator.

[0093] In the present embodiment, a wavelength plate having aretardation value of 0.07 wavelength is used for the retardation plates12 and 13 such that the orientation of the wavelength plate isperpendicular in the retardation plate 12 and the retardation plate 13.Thereby, an optical phase compensation of +0.07 wavelength is achievedfor the land 7 d and an optical phase compensation of −0.07 wavelengthis achieved for the groove 7 e. An optically uniaxial crystal such ascalcite, quartz or LiNbO₃ may be used for the retardation plates 12 and13. In addition, it is also possible to use a 0.07-wavelength platehaving a thickness of about 1 mm in which two quartz plates are bondedwith each other in a relationship such that the crystal axes thereofintersect perpendicularly with each other.

[0094] TABLE I below shows the result of measurement of CNR(carrier-noise ratio) and the cross-talk for the magneto-opticalrecording and reproducing apparatus of the present invention. TABLE Ipresent invention comparative exp. CNR land  45.3 dB  42.1 dB groove 45.8 dB  42.3 dB Cross-talk land −30.2 dB −10.3 dB groove −30.0 dB−11.1 dB

[0095] Referring to TABLE I, the measurement was made by first erasinginformation from a selected land 7 d and two adjacent grooves 7 e atboth sides of the selected land 7 d, writing information onto theselected land 7 d, and reading the information by using the detectionsystem for the land. The measurement for the groove 7 e is conductedsimilarly, by exchanging the land 7 d and the groove 7 e. In themeasurement, the magneto-optical disk 7 is driven at a linear velocityof 5 m/sec and a laser power of 1.5 mW was used on the recording medium7. In the recording of information, a recording mark having a length of0.45 μm was recorded under the existence of modulation magnetic field of±15000e, by irradiating a laser beam having a pulse duty of 40% and anoptical power of 7.5 mW in synchronization with the modulation magneticfield.

[0096] In the experiment of TABLE I, the measurement of cross-talk wasconducted as follows.

[0097] In the case of measuring the cross-talk on the signal recorded ona selected lank 7 d, the information on the selected land 7 d as well asthe information recorded on the grooves 7 e at both sides of theselected land 7 d are first erased, and recording of information isconducted on the selected land 7 d. Next, a measurement is made on asignal output level CL reproduced by the signal detection system for theland, while tracking the land 7 d on which the recording of theinformation has been made previously. It should be noted that the signaldetection system for the land includes the photodetectors 20 and 21 andthe amplifier Amp1 of FIG. 7.

[0098] Next, a tracking is made for each of the adjacent grooves 7 e,and a measurement is made on a signal output level CR reproduced by thesignal detection system for the groove, for each of the adjacent grooves7 e. It should be noted that the signal detection system for the grooveincludes the photodetectors 22 and 23 and the amplifier Amp2. Thereby,the cross-talk is calculated as a difference between signal level CR andthe signal level CL (CR-CL), wherein the groove 7 e that provides alarger output level CR is used in the foregoing calculation of thecross-talk. Further, the measurement of the cross-talk for a groove 7 eis conducted similarly as above, by exchanging the land 7 d and thegroove 7 e.

[0099] In the foregoing measurement of the cross-talk, it should benoted that a recording mark having a length of 1.35 μm is used.Otherwise, the measurement was conducted similarly to the measurement ofthe CNR.

[0100] Further, TABLE I above includes result also for a comparativeexperiment, in which the retardation plates 12 and 13 are removed fromthe composite optical element 9.

[0101] Referring to TABLE I, it can be seen that the present inventionachieves an increase of the CNR of +3 dB as compared with thecomparative experiment, wherein the increase of +2 dB is attributed tothe contribution of the elimination of the zeroth-order beam 27 shown inFIG. 9, while the increase of +1 dB is attributed to the increase of thecarrier level in the reproduced signal as a result of the optical phasecompensation.

[0102] Further, TABLE I indicates that the cross-talk is suppressed to−30 dB.

[0103] Next, a description will be made on the recoding and playbackmargin for the magneto-optical recording apparatus of the presentinvention.

[0104]FIG. 10 shows the relationship between the Jitter and the opticalpower used for writing information, wherein FIG. 10 represents theresult of measurement of the jitter of a 2T signal for the case in whicha random signal of the RLL1-7 format is recorded both on a land 7 d andan adjacent groove 7 e with a minimum mark length of 0.45μ. In themeasurement of FIG. 10, it should be noted that the linear velocity ofthe magneto-optical disk 7 is set to 5 m/sec at the time of reading ofthe information, and the reading of the information is achieved bysetting the laser power to 1.5 mW on the disk 7.

[0105] As can be seen in FIG. 10, a jitter of less than about 8% isachieved when the write optical power is set to a range between 7 mW and10 mW. The result of FIG. 10 indicates that a recording density of 3.2Gbit/inch² can be achieved by the magneto-optical information recordingand reproducing apparatus of the present invention.

[0106] Thus, the present invention provides a magneto-opticalinformation recording and reproducing apparatus as well as an opticalinformation detection system used therein wherein the cross-talk betweenthe lands and grooves on the magneto-optical recording disk is minimizedand wherein the interference between the recording marks aligned on atrack is also minimized.

[0107] Further, it should be noted that, while the present invention hasbeen described for a magneto-optical information recording andreproducing apparatus, the present invention is by no means limited tosuch a specific apparatus but is applicable also to ROM disks,write-once-read-many disks and phase transition disks in which phasepits are formed.

[0108] Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

[0109] The present application is based on Japanese priority applicationNo. 10-23147 filed on Feb. 4, 1998, the entire contents of which arehereby incorporated by reference.

What is claimed is:
 1. An optical information detecting apparatusoptically detecting information recorded on a recording medium, saidoptical information detecting apparatus being adapted to hold saidrecording medium and comprising: a beam source; a first optical systemfocusing an optical beam produced by said beam source on a recordingsurface of said recording medium; a photodetection unit; and a secondoptical system directing an optical beam, produced as a result ofirradiation of said optical beam on said recording surface, to saidphotodetection unit, said second optical system including a beamdividing element disposed so as to incident said produced optical beam,said beam dividing element extracting, from said produced optical beam,a plurality of optical beam elements traveling generally parallel witheach other in said reflection optical beam, by dividing said producedoptical beam such that said plurality of optical beam elements reachsaid photodetection unit along respective optical paths.
 2. An opticalbeam detecting apparatus as claimed in claim 1 , wherein saidphotodetection unit includes first and second photodetectors onrespective optical paths of first and second optical beam elementsincluded in said produced optical beam for detecting an informationsignal recorded on a land formed on said recording surface of saidrecording medium and wherein said photodetection unit includes third andfourth photodetectors on respective optical paths of third and fourthoptical beam elements includes in said produced optical beam fordetecting an information signal recorded on a groove formed on saidrecording surface of said recording medium.
 3. An optical informationdetecting apparatus as claimed in claim 1 , wherein said beam dividingelement has a unitary body.
 4. An optical information detectingapparatus as claimed in claim 1 , wherein said beam dividing elementapplies a first optical phase compensation to a first optical beamelement included in said plurality of optical beam elements and a secondoptical phase compensation to a second, different optical beam elementincluded in said plurality of optical beam elements.
 5. An opticalinformation detecting apparatus as claimed in claim 4 , wherein saidfirst optical phase compensation and said second optical phasecompensation act oppositely and have the same magnitude.
 6. An opticalinformation detecting apparatus as claimed in claim 5 , wherein saidfirst optical phase compensation includes an optical phase shift to saidfirst optical beam element by +0.07 times a wavelength of saidreflection optical beam and wherein said second optical phasecompensation induces an optical phase shift to said second optical beamelement by −0.07 times a wavelength of said reflection optical beam. 7.An optical information detection apparatus as claimed in claim 4 ,wherein said beam dividing element includes a first optical phasecompensation plate of an optically uniaxial crystal and a second opticalphase compensation plate of an optically uniaxial crystal, said firstoptical phase compensation plate and said second optical phasecompensation plates being disposed such that an optical axis of saidfirst optical phase compensation plate and an optical axis of saidsecond optical phase compensation plate intersect perpendicularly.
 8. Anoptical information detecting apparatus as claimed in claim 7 , whereinsaid first optical phase compensation plate and said second opticalphase compensation plate are jointed side by side.
 9. An opticalinformation detecting apparatus as claimed in claim 4 , wherein saidfirst optical phase compensation compensates for an optical phase ofsaid first optical beam element produced by a land formed on saidrecording surface and wherein said second optical phase compensationcompensates for an optical phase of said second optical beam elementproduced by a groove formed on said recording surface adjacent to saidland.
 10. An optical information detecting apparatus as claimed in claim7 , wherein said beam dividing element further includes a firstpolarization beam divider provided on said first optical phasecompensation plate and a second polarization beam divider provided onsaid second optical phase compensation plate, said first polarizationbeam divider switching an optical path of said first optical beamelement between a first optical path and a second optical path inresponse to a polarization state of said first optical beam element,said second polarization beam divider switching an optical path of saidsecond optical beam element between a third optical path and a fourthoptical path in response a polarization state of said second opticalbeam element.
 11. An optical beam detecting apparatus as claimed inclaim 2 , further including a first differential amplifier connected tosaid first and second photodetectors for detecting a difference inoutput of said first and second photodetectors as said informationsignal recorded on said land and a second differential amplifierconnected to said third and fourth photodetectors for detecting adifference in output of said third and fourth photodetectors as saidinformation signal recorded on said groove.
 12. An optical beamdetecting apparatus as claimed in claim 10 , wherein said firstpolarization beam divider and said second polarization beam divider aredisposed with a separation from each other.
 13. An optical beamdetecting apparatus as claimed in claim 12 , wherein said first andsecond polarization beam splitters are separated with a distancecorresponding to a zeroth-order diffraction beam produced by said landand groove on said recording surface of said recording medium andforming a part of said reflection optical beam.
 14. An optical beamdetecting apparatus as claimed in claim 13 , wherein said first andsecond polarization beam splitters are disposed so as to intercept twofirst-order diffraction beams produced in said reflection optical beamat both sides of said zeroth-order diffraction beam.
 15. An optical beamdetecting apparatus as claimed in claim 10 , further comprising a prismstructure between said first and second polarization beam dividers fordirecting said zeroth-order beam to said photodetection unit for a servocontrol of said first and second optical systems.
 16. An optical beamdetecting apparatus as claimed in claim 15 , wherein said prismstructure includes a first wedge prism having a first prism surface anda second wedge prism having a second prism surface disposed with amutual separation such that said first and second prism surfaces are ina mutually inclined relationship, said first and second prism surfacesdirecting said zeroth-order beam respectively to a corresponding fifthphotodetector and a corresponding sixth photodetector of saidphotodetection unit along fifth and sixth optical paths for a trackingservo control of said first and second optical systems.
 17. An opticalbeam detecting apparatus as claimed in claim 16 , further including athird wedge prism having a third prism surface and a fourth wedge prismhaving a fourth prism surface disposed side by side in a state that saidthird and fourth prism surfaces are inclined in mutually oppositedirections, said third and fourth prism surfaces directing saidzeroth-order beam to a photodetector array of said photodetection unitincluding four photodetectors arranged in a four-quadrant formation fora focusing servo control of said first and second optical systems. 18.An optical beam detecting apparatus as claimed in claim 1 , wherein saidbeam dividing element includes: first and second optical phasecompensation plates disposed side by side to form a unitary substrate;first and second Wollaston prisms provided on said first and secondoptical phase compensation plates respectively with a mutual separationfrom each other in a first direction; first and second wedge prismsrespectively provided on said first and second Wollaston prisms, saidfirst and second wedge prisms having respective prism surfaces inclinedwith each other; third and fourth wedge prisms disposed on said unitarysubstrate between said first and second Wollaston prisms with aseparation from each other in a second direction perpendicular to saidfirst direction, said third and fourth wedge prisms having respective,mutually inclined third and fourth prism surfaces; and fifth and sixthwedge prisms provided on said unitary substrate between said third andfourth wedge prisms, said fifth wedge prism having a fifth prism surfaceinclined to said third wedge prism, said sixth wedge prism having asixth prism surface inclined to said fourth wedge prism.
 19. An opticalbeam detecting apparatus as claimed in claim 1 , wherein said recordingmedium is a magneto-optical disk, and wherein said optical beamdetecting apparatus further includes: a turn-table adapted to hold saidmagneto-optical disk thereon; a spindle motor rotating said turn-table;and a magnetic head disposed adjacent to said magneto-optical disk at aside opposite to said recording surface.